This invention relates to high purity water, and more particularly it relates to production of high purity water using reverse osmosis.
In producing a high purity water product using reverse osmosis membranes, it is difficult to reject gases such as carbon dioxide and/or ammonia. The carbon dioxide and ammonia gas pass through the reverse osmosis system and re-establish an equilibrium in the product water. This, of course, adversely affects the product water quality, e.g., resistivity.
Many processes have been suggested to provide high purity water. For example, U.S. Pat. No. 5,178,766 suggests the use of a composite semipermeable membrane comprising a microporous substrate; and a cross-linked polyamide-based ultra-thin membrane superposed on the microporous substrate, the ultra-thin membrane having covalently bonded quaternary nitrogen atoms. When such a membrane, i.e., cross-linked polyamide-based composite semipermeable membrane, is used, it is suggested that a double pass reverse osmosis system is sufficient to produce high purity water. That is, a regenerative ion-exchanger normally used may be eliminated.
U.S. Pat. No. 5,670,053 discloses a process for purifying water including removing cations, anions and carbon dioxide and/or ammonia from water feed stream to produce high purity water having a resistivity of greater than 1 megohm-cm comprising the steps of providing a water feed stream to be purified. In this system, a gas permeable hydrophobic membrane is provided between a first reverse osmosis system and a second reverse osmosis system for removing carbon dioxide and/or ammonia gas through a gas permeable hydrophobic membrane.
U.S. Pat. No. 5,061,374 discloses an improved deionized water treatment system and method that consists of using a first and second water treatment train wherein the second train improves water quality by reducing particulate and total organic carbon by means of a reverse osmosis membrane assembly. The final pure water has particulate sizes less than one micron and a total organic carbon measure of less than one part per billion.
U.S. Pat. No. 4,347,704 discloses that the costs of water used in a fossil fuel fired power station are reduced by a process which employs multiple reverse osmosis stages. The process also employs station waste heat to concentrate solid waste material to facilitate disposal thereof. In this process, concentrate from a first reverse osmosis is directed to a second reverse osmosis and concentrate from the second reverse osmosis is directed to a third reverse osmosis. Steam is used to heat the concentrate from the third reverse osmosis and solid matter is extracted therefrom.
U.S. Pat. No. 4,980,066 discloses an improved membrane separation system which comprises a plurality of discrete membrane separation units, each membrane separation unit comprising at least two membranes and also comprising an inlet for process fluid and optionally an outlet for process fluid. Each membrane separation unit is located within a container for permeate, the container comprising a permeate outlet and wherein the container is such as to enable the membrane separation units to be at least partially immersed in permeate.
U.S. Pat. No. 5,651,894 discloses an ultrapure water purification scheme that incorporates the use of certain reverse osmosis filtration membranes that are especially susceptible to oxidative attack. The scheme employs the combination reducing/metals scavenger/biocidal agent dithiocarbamate as a pretreatment additive to maintain a nonoxidizing environment, to bind contaminant metals to improve their removal from source water, and to reduce microbial content. In addition, the generation of oxidants by cavitation effects is avoided by a reduced pressure retentate recycle. Both measures prevent oxidative attack and fouling of the reverse osmosis membranes.
U.S. Pat. No. 5,128,041 discloses microporous membranes having positive zeta potentials in alkaline pH which are provided by radiation grafting diallyl-dimenthylammonium dimethyl ammonium chloride to the surfaces of a microporous, polyamide membrane using ionizing radiation. The membranes have particular use in microelectronics manufacture where quick rise up times with ultrapure water are required.
Thus, it will be seen that there is a great need for a process which can be tuned to the system, including feedwater and the use of different membranes, and which will consistently produce high resistivity water on a continuous basis, even with changing composition of the feedwater.